[FEATURE] Promote existing sessions to Agent Team teammates (session → teammate with preserved context)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Agent Teams currently require spawning new teammates from scratch — teammates start with a blank context and no prior conversation history. There is no way to take two existing sessions that have already accumulated deep context (e.g., research findings, analysis, drafts) and connect them as teammates so they can communicate directly.

Real-world scenario: I have Session A (a research agent that spent hours finding and analyzing 30+ papers on AI security) and Session B (a writing agent drafting a paper that needs to cite those papers). Both sessions have rich, accumulated context. I want them to directly discuss which papers to cite, with the researcher explaining relevance and the writer asking about fit for specific sections — like two coworkers in a Slack channel.

Current workarounds and why they fall short:

1. Manual copy-paste — Loses structure, tedious, breaks flow
2. Shared file as message board — No real-time dialogue, no awareness of each other
3. Export summary → new Agent Team — Loses the nuanced context that makes each session valuable. A 70MB session distilled to a summary loses the details that matter most during collaborative discussion
4. Session Bridge (3rd-party plugin) — Only supports request-response pattern, locks the listening session, not true bidirectional dialogue

Proposed Solution

Allow existing sessions to be promoted (or "joined") into an Agent Team as teammates, preserving their full conversation context.

Possible UX:

# In Session A (or a new Team Lead session):
> /team create

# Promote existing sessions by ID:
> /team join <session-B-id>
> /team join <session-C-id>

# Or from within each session:
> /team join <team-id> --as "Researcher"

What this enables:

• Each promoted session retains its full accumulated context (research findings, analysis, code understanding, drafts)
• Teammates can use the existing mailbox/messaging system to communicate directly
• The Team Lead can coordinate, or teammates can have autonomous discussions
• Users can organically evolve from independent exploration into collaborative synthesis

Technical considerations:

• Session history is already stored as JSONL files in ~/.claude/projects/
• The session's context window state would need to be carried over (or reconstructed from the JSONL) when the session transitions to teammate mode
• Context compaction may be needed if the session is already near the context limit, since teammate messaging overhead needs headroom
• Need to handle the transition cleanly — the session should stop accepting direct user input and switch to teammate mode (receiving messages via mailbox instead)

Why this matters

The most valuable collaboration happens when both parties bring existing expertise and context to the conversation. Today, Agent Teams only support the "hire new employees and brief them" pattern. This feature would enable the "bring your senior specialists into a room together" pattern — which is fundamentally more powerful for complex knowledge work.